Circuit for oscillograph with provision for calibrating during operation



July 8, 1952 ELLIS CIRCUIT FOR OSCILLOGRAPH WITH PROVISION FOR CALIBRATING DURING OPERATION Filed May 9, 1950 FIG].

AMPLIFIER INVENTOR BY \X Qwq ATTORNEY Patented July 8, 1952 I FOR OSCILLOGRAPH WITH PRO- VISION FOR CALIBRATING DURING OR amnion I Greer Ellis, Pelham, N. Y. 7 Application May 9, 1950, Serial No. 160,943

' 7 Claims. (01. 175-183) ra'yfmovingcoil or othertypes of' oscillograph screen record strip'si'jTh'e' invention will be described as combined'with 'an oscillograph having a'sweeprayand ascreen.

It' is an objectof; this invention to provide'circuits with suitable impedances'and switch means that give improved calibration features. More particularly, the circuit of this invention provides for calibration at anytime during a test and it is not dependent-upon the maintenance of a standardized voltage or 'the balancing of a bridge circuit."

If the impedance of the signal circuit is a resistance, the invention usesresistances as calibration impedance's. Similarly, capacitance or inductance impedances in the 'signalcircuit are matched by calibrationimpedances of the same kind but of difierent value. Combinations of different kinds of impedances' for the'signal circuit are matched by similar combinations for calibration impedances f The inventionoperate's equally well with signal circuits of resistance, capacitance, inductance or any impedance combinationsrsuch as are employed in the known state of the art relating to such circuits. J

In accordance with another feature of the invention, the transducer circuit is combined with an amplifier and oscillograph with a vibrator switchor other motor-operated switch connected in series with acalibrating circuit to produce a second line onjtheoscillograph for indicating calibration at any time during the use of the apparatus.

Another feature of the invention relates to the connection of a power-operated] vibrator switch in the circuit with an oscillographwith provision for preventing extraneous" electrical noise from the vibrator drive circuit from getting into the measuring circuit that drives the oscillograph.

Other objects, features and advantages of the invention will appear orjbepointed out as the description proceeds.

In' the drawing, forming "a part hereof, in which like reference characters" indicate corresponding parts in all thejvie'w's, jf I Fig. 1 is'a diagrammaticflview showing an example .of this inventioncompri'sing a simple unbalanced resistance transducer circuit combined with an amplifier and an oscillograph.

n Figs. 2 to 5 are 'diagrams'gshowing the calibra- .2 tion produced by the apparatus shown in Fig. 1.

Fig. 6 is a view similar to Fig. 1 but showing a modified form of the invention.

The signal circuit of this example of the invention includes a blocking resistor Ill connected with a source of power, comprising a battery 12, and having terminals l4 and 15 across which a age I! or other variable load is connected. This load, the variations of which are to be measured, may be a strain gage, or any resistance transducer for measuring'displacement, load, pressure, or other physical conditions. i

It is evident thatlthe resistor I0 is necessary as a ballast resistor in order to obtain signals from variations in the gage l1. ballast resistor, the voltage measured across the gage I! would be merely the terminal yoltage of the battery |2. Y

The terminals l4 and I5, also connect with an amplifier 19 that supplies, power to the oscillograph 20 through suitable conductors. The amplifier I9 in the example is, an ialternating current amplifier using electronic tubes. The oscillograph is a cathode ray instrument having provision for sweeping the ray across the screen at an adjustable andcontrolled freqency. The construction of such amplifiers'and.oscillographs are .well understood in the' art, and a detailed description of them is not necessary for a complete understandingv of this example of the invention.

The terminal IA. of the signal circuit is connected with a calibration circuit including a switch arm 22. Switch contacts 251-29. inclusive, of the calibration circuiteach connect with a precision calibration resistor of different value. These resistors are designated by the. same reference characters astheinassociated contacts 25-29 but with a prime appended, and they are connected with a common terminal 33 of a vibrator switch indicatedby the-general referencecharacter 35. V v a The terminal 33 of the vibrator connects with a fixed contact 36 in a housing 3'! that encloses the vibrator, and there is another fixed contact 38 in the vibrator witha terminal 39. A reed 4| vibrates between the contacts .36, and 38 and touches the respective contactsflat opposite ends of its stroke. plied by an operating coil '43 of the vibrator switch. This coil is in theicircuitl of the fixed contact 38 and is connected. ,l'outside of the vibrator housing with a batter ;.44; There is a manually operated switch '45jin'iseries with the of the vibrator switch. 1 v.

The reed 4| has a terminal, at its fixed end,

battery for.starting and stopping the operation 7 Without this Motivepower for the reed is sup- 44. The reed is thus caused to vibrate, in a well known manner, by the power supplied from the battery 44.

The terminal 46 of the vibrator'switch 35-is connected with the terminal of the bridge so that with the parts in the positions shown in Fig. 1, there is a circuit from the terminal M of the bridge to the arm 22, and through thecontact 28, resistor 28', vibrator terminal 33, fixed contact 36, reed 4|, and terminal 46 of the vibrator back to the terminal l5 of'the bridge. Any one of the'resistors 25',126.', '21 M29 can be connected across theresistance transducer in place 01 the resistor 28', by merely moving the arm 22 into position to touch a different one of 'the' contacts 25-29.

In order to'prevent extraneous electrical noise from the drive circuit of the vibrator switch "from' getting into the measuring circuit to the amplifier 4 Calibration signals add directly to any signals coming from the transducer. It is to be understood that in normal practice the change in total impedance by addition oi the calibration impedance, is sufliciently than the impedance or the, transducer sbithat'tthej'iiornrfl operation of the transducer is not sensibly affected by the calibration operation.

Figure 2 shows the calibration of the apparatus when there is nqsignal from the transducer II,

that is, the transducer I1 is in its original condition or subject to a static load. Two lines 5| and- 52 are seen on the oscillograph screen. The upper line 5| is produced by the sweep of the ray across the'screen while the calibration circuit through the vibrator switch 35 is open; and the lower line 52 is produced by the sweep of the ray across the screen during" the time that the calibration circuit through the vibrator switch 35 is closed.

The spacing of the'lines 5| and'52 from one another, that is, the'calibration or the apparatus,

-- depends upon which'ot the'calibra'ti'on resistors i9, there is a filter 48 jconnected'across the terminals 39 and 45 of the vibrator switch 35. In order to minimize extraneous currents coupling into the conductors that arecommon to the measuring circuit and the driving circuit of the vibrator switch, it'is important to have the filter 48 connected directly to the terminal 46. The filter is less effective if it is connected at some intermediate point' along the conductor that leads from the terminal 46 to the battery 44 orfif connected with some intermediate point'along the conductor that connects the'vibrator terminal 45 with the bridge terminal |5.

Anothe'r'ifeature tor reducing extraneous electrical noise is thelocationbf the vibrator. drive contact 38 on'the opposite side of the reed from the resistor connecting contact 35'. these contacts on opposite sides of the reed and having contact 38'open beforecontact 36 closes, the battery circuit that operates the reed of the vibrator switch 'is always open when the circuit through the calibration resistors 29 is closed. This feature is not essential, however, and when noise level is not critical, the vibrator switch can be made with the contact 38 on the same side of the reed as the contact and with the motor coil 43 arranged to pull the reed away from both of the contacts.

The essential feature'of the vibrator switch 35 is that it opens and closes the circuit through the calibration resistors 25-29 automatically and with a predeterminedcontrolled or regular frequency. The vibrator switch 35 is merely representative of switch means that open and close this circuit through the resistors 25'-29 automatically with a predetermined frequency and in accordance with a controlled cycle of operation.

When the circuit through the'vibrator switch 35 is open, the amplifier I9 is supplied with the full voltage across the resistance transducer H, and any changes in the resistance of the transducer i1 cause corresponding changes in the input voltage of the amplifier l3.

Connecting a high resistance, such as the resistor 29', in parallel'with the resistance transducer i1, induces a change in voltage across the terminals i4 and i5 proportional to the total change in resistance in identical mannerto change in transducer resistance.

By having 2549' is. connected inseries with the vibrator switch.35.' The lines 5| and52'are actually made by the same ray which movesup and down with each opening'and closing or the vibrator switch 35, but in Fig. 2 the sweep of the oscillograph ray is not'in phase with the oscillations of the vibrator 35 and the vertical lines'between the lines 5| and 52 are, therefore, haphazard and not visible.

Fig. 3 shows the'condition'that is obtained when the sweep of'the oscillograph is in phase with the oscillations of'the vibrator switch 35. There are upper horizontal lines 54 corresponding to the line 5| of Fig. 2, and there are lower horizontal lines '55 corresponding to the line 52 of Fig. 2. Because of the fact that the ray moves Fig. 2, Fig. 3 shows results obtained with no load or a static load' on the transducer.

Fig. 4 shows a number of short lines 6| on the oscillograph screen representing signals from the transducer |1 when subject'to varying or vibrating load. These lines 5| would joi'ri'into a continuous line if the vibrator switch were not used, but the amplitude of the vibrations in 4 would be of no significance unless the observer knew the calibration or the, apparatus. This -calibration can be determinedby closing the manually -oper'ated Switch 4550 that the vibrator switch comes into operation and produces a series of lines 52 corresponding to each closing of the circuit through one of the calibration resisamplitude of the oscillograph indication, the

calibration of the apparatus can also'be changed without making any change in the load, or the strain gage or other transducer, merely by moving the calibratingarm 22 oneiway or the. other to obtain a reasonable amplitude of the calibration signal for best calibration purposes;

The calibration system'c'an be applied to any section of the signal'circuit and its ei fect on the transducer arms can" becomp'uted' from the known constants of thefbr'idge.

Figure 5 shows conditions simi ar to Fig. 4 but with the signal rrequency substantially higher than the, frequency of the calibration switch. .As in the1case:of ]ig.,2, two separate lines 64 and 65 are obtained'and the vertical lines connecting. them are haphazard and not visible. The upper line 6 4 corresponds to the circuit conditions when the calibrating switch 35 is open, and the lower 1ine 65 correspondswith the circuit conditions when the calibrating switch 35 is closed.

Fig, 6 shows an alternative method for produc-- ing acceptable calibration signals. A small value of suitable calibration impedance is introduced in series with the transducer. This can be done,

as shown in Figure 6, by alternately shorting out.the selected calibration value. Because of more critical circuit conditions, such as an error from switch contact resistance in these low impedance circuits, the paralleling method previously describedis usually preferred.

In Fig. 6 the calibration impedances are inserted in series with the transducer [1. This figure shows terminals I4 and IS with conductors 64 and. 65 for connection with the oscillograph through anamplifier in the manner similar to Fig. 1. Between the terminal l5 and the transducer ll, however, there is a switch element 68 that closes the circuit between the transducer l1 and the terminal [5 when in the solid line position shown. This switch element 68 can be moved into the dotted line position shown in the drawing, and when in such position, it connects the terminal I5 with a conductor leading to a group of calibration impedances designated generally by the reference character 12. An arm 13 moves about'a center" terminal 14 and can be shifted to connect the terminal with any selected one of the impedances I2.

The terminal 14 is connected by a conductor 16 to a switch contact 18 which the switch element 68 touchs when in the full line position shown in the drawing. The terminal 18 is also connected with the terminal 33 of the vibrator switch or chopper 35.

The vibrator switch or chopper 35 has a filter 48 connected across its terminals 39 and 46, as

in the combination shown in Fig. l. The power circuit for the operating coil of the vibrator switch-35 has a'battery 44 and is otherwise similar to the power circuit-of the vibrator switch of Fig. 1 except that the circuit shown in Fig. '6 has a manually operated switch element 80 which is connected to the switch element 68 by a mechanical1inka e82. 1 1

.When the-switch elements 68 and 80 are in their full line positions, the transducer llis connected directly .with the terminal l5 and both the chopper 35 and calibrating impedances'lZ are out of the circuit. When the switch elements -68 and 80 are shifted intotheir dotted line positions, the transducer I1 is connected in series with a selected calibrating impedance of the impedances 12 through switch contact l8,conductor 16, arm 13 and conductor ll] which carries the circuit back to the contact 15. The vibrator switch or chopper 35 is connected in parallel with the calibration impedances 72 through a conductor 85 that joins the center terminal 14 with the'chop'per ,terminal-33, and through a conductor 86 that joins thecenterterminal 46 of the chopper 35 to the conductor 1|].

When the reed dlof the chopper 35'touches the contact 33, the calibration impedances 12 are short circuited and only the transducer I1 is across the terminals l4 and I5. Whenever the reed M is away from the contact 36, a selected impedance of the calibration impedances 12 is connected in'series with .they transducer [1. across therterminals l4 andIS-..

J-An important requirement of-this invention is.

The preferred embodiment of the invention has beenillustrated and described,.but changes and modifications canbe made andsome features can be used .aloneoriin diiferen't combinations withoutdeparting. from theinvention as defined in the claims.- 1 h Pt What is claimed is: t

- 1.,Measuring' apparatus comprising a circuit, including an impedance transducer in series with another impedance, connections at opposite ends of the circuit through. which power is supplied to the circuit for. causingthe impedance transducer to generate signals, a calibration impedance of a predetermined known value and of a type and size that gives calibration signals comparable to the transducer signals when inserted in the transducer circuit and supplied with power from the same source as said connections at the opposite ends 'of. the circuit, consistent repetitive switching means that move into one position to remove the calibration impedance from the transducer circuit so that the signals generated are the efiect of. the transducer on the power supplied without effect from the calibration impedance and that move-into another position to insert said impedance-into the transducer circuit so that the signals generated by the transducer are modified by the impedance, and conductors through which signals in the circuit supply power to an oscillograph independently'of the position of said ducer to generate ignals, a calibration impedance of a predetermined'known value and of a type and size that gives calibration signals comparable to the transducer signals when inserted in the transducer circuit and supplied with-power from the same source as said connections at the 0pposite ends of the circuit, consistent repetitive switching means comprising a vibrator in series 3 with the calibration impedance, a motor coil that operates the vibrator, a movable element in the vibrator that opens and closes the circuit of the calibration impedance and the motor coil circuit, a powersource connected with the motor coil and a filter connected in the motor coil circuit parallel" with the-portion 'of the motor coil circuit that opens and closes during the operation of the vibrator. .1 I

' .3. Apparatus for supplying signals to power an oscillograph,said apparatus comprising an impedance transducer, another impedance connected in series with the'transducer, connections at opposite ends of'this series circuit to supply power to the circuit forcausing the transducer to generate signals, a calibration impedance of a predetermined known value, switch means by which the calibration impedance is connected in the transducer circuit in position to modify the signals from that circuit, when the calibration impedance is supplied with power from the same series as the transducer and simultaneously with the supply of power to the transducer, said switch means including a vibrating reed fixed at one end, two relatively fixed contacts on opposite sides of the vibrating portion of the reed, a motor coil that causes thereed to oscillate consistently back and forth from one contact to the other, the motor coil being connected with one of the contacts and with a source of power, three terminals at one end of the vibrator switch, one of the terminals being located adjacent to the fixed end of the reed and each of the other two terminals being connected with a diflerent' one oi! the contacts between which the reed moves, a conductor connecting the contact opposite to the motor coil contact with the calibration impedance, conductors connecting the reed terminal of the vibrator switch with the motor coil power supply and with the transducer circuit, and a filter comprising a resistor and capacitor in series and connected directly with the reed terminal and the motor coil terminal of the vibrator switch.

4. Test apparatus comprising an impedance transducer that is subjected to the force to be measured, another impedance in series with the transducer, connections at opposite ends of said series circuit and through which power is sup.- plied to the circuit for causing the transducer to generate signals, a plurality of calibration impedances of different and predetermined values, switch means movable into different positions to introduce diiierent ones, of the calibration impedances into acalibration circuit, conductors by which the calibration circuit is connected with the transducer circuit to receive signals from the same source of power that energizes the transducer circuit, and consistently repetitive switching means connected in series with the impedances and movable into diiferent positions to open and close the calibrationimpedance circuit.

5. Measuring apparatuscomprising an impedance transducer, another impedance in series with the transducer and forming therewith a signal generating circuit, connections at opposite ends of said circuit through which power is supplied to the circuit for causing the transducer to generate signals, a calibration impedance of predetermined value I located in a calibration circuit, conductors by; whichthecalibration circuit is connected in the transducer circuit, consistently repetitive switching means that alternately connect the calibration circuit with the transducer circuit and then disconnect the calibration circuit from the transducer circuit, said switch means comprising a vibrator in series with the calibration impedance and having a reed fixed at one end, two relatively fixed contacts on opposite sides of the vibrating motor coil that oscillates the reed back and forth into and out of contact with said relatively fixed contacts, said coil being connected with one of the contacts and with a battery, terminals located at one end of the vibrator switch and one of which is adjacent to the fixed end of the reed and one of which is connected with the, relatively fixed contact that is in the vibrator coil circuit. and a conductor that is outside of the power supply circuit to the oscillograph and t at. connects the motor coil circuit to the terminal that is adjacent to the fixed end of the reed.

" 6. Apparatusforimeasuring voltage changes, said apparatus including two terminals across which is to be connected a circuit in which the voltage change is to be-measured, a blocking impedance in series with the circuit to be measured, a power source connected across the series connected blocking impedance and circuit to be measured, a cathode ray oscillograph connected across said terminals through an amplifier for indicating a line representing a normal voltage across the terminals, a calibration circuit also connected across said terminals and deriving power from the same source-as said circuit in which the voltage change is to be measured, a plurality of calibration impedances each of which is of predeterminedvalue in the calibration circuit, switch means inthe calibration circuit and in series with the calibration impedances, said switch means having a bias toward open position, a motor that operates the switch means to close and open the calibration circuit with consistently repetitive operation, a manually-open ated control for stopping and starting the motor to make the calibration circuit effective to produce a second line on the oscillograph, and an adjustable conductor movable into different positions to connect different calibration impedances in series with the switch means so as to control the spacin of the lines on the oscillograph.

7. Apparatus for measuring voltage changes, said apparatus including two terminals across which is'to be connected a circuit in which the voltage change is to be measured, a blocking impedance in series with the circuit to be measured, a power source connected across the series blocking impedanceand circuit to be measured, a cathode ray oscillograph connected across said terminals through anamplifier for indicating a line representing a normal voltage across the terminals, a'calibration circuit also connected across said terminals, a plurality of calibration impedances each of which is of predetermined value, an alternating-current amplifier connected across the series circuit, an oscillograph to which the output of the amplifier is supplied, the calibration circuit being connected in parallel with said circuit to be measured and supplied from the same power source that supplies said circuit to be measured, a movable conductor for connecting selected impedances in the calibration circuit, a switch in series with the selected calibration impedance, motor means that opens and closes the switch in accordance with a consistently repetitive movement, and a controller operable to start the motor means.

GREER ELLIS.

REFERENCES CITED The following references are of record in the file Of this patent:

UNITED STATES PATENTS 

